1997 GMC Sierra Brakes, pedal is unstable.?

While driving this morning, I stopped normally at a sign, and then the pedal suddenly went down like the first section of the master cylinder failed. I bought a new master cyl, installed it, bled the system complete. Now it stops, but if, while sitting, you press hard and steady on the pedal, engine running, it will go down. Booster is not leaking down, pedal stays up with engine switched off. Darn??? Is there part of the ABS that would cause this? I thought it was the booster, but there is no vacuum leak. Is it possible for the booster to go bad without leaking vacuum?. It looked like there was brake fluid on the big vacuum hose connection, but the old cyl didn't appear to have any leaks. No leaking calipers or wheel cylinders. What to can be wrong? Any expert opinions? No idiot lights are on. Thanks.